This invention relates to a process for manufacturing silicon powder with a particle size of less than 350 .mu.m, with catalytic properties for the manufacture of alkyl or aryl-halosilanes, intended for the synthesis of silicones.
The synthesis of alkyl or aryl-halosilanes by reaction of a halogenated hydrocarbide, for example methyl chloride, on silicon in the presence of copper at between 250 and 350.degree. C. is known from U.S. Pat. No. 2,380,995 issued to E.G. Rochow in 1945.
This reaction has been industrially developed to a large extent, since the silicone industry is based on it. It is usually used with CH.sub.3 Cl and leads to a mixture of various methyl-chlorosilanes in variable proportions.
The role of copper in the reaction was described 20 for example in the article by P. Trambouze, "Contribution a I'etude du mecanisme de la synthese des methyl-chlorosilanes--Contribution to the study of the methyl-chlorosilanes synthesis mechanism," Bulletin of the Chemical Society of France, No. 288, 1956, pp. 175625 1765, and in the article by G. H. Kolster, "The v' and .epsilon.' phase of the system copper-silicon as solid reactants in the synthesis of dimethyl-dichlorosilane," Compilation of Chemistry publications, vol. 83, 1964, pp. 737-751. These publications show the catalytic role of the Cu.sub.3 Si phase in the chloromethylatlon reaction.
However, the applicant's patent FR 2720385 shows that when increasing quantities of copper are added into silicon, other phases such as Cu.sub.4 SiAl, Cu.sub.2 Si.sub.3 Al or Cu.sub.2 Si.sub.3 Ca, appear before Cu.sub.3 Si which makes it necessary to add slightly greater quantities of copper than are used in the active Cu.sub.3 Si phase. Thus, only a fraction of the added copper is used.
Dow Corning's European Patent No. 0604092 describes the preparation of a silicon-copper alloy containing from 0.01 to 9% of copper for the preparation of halosilanes and recommends that only part of the copper should be added to the alloy, the rest being added in the normal manner using a copper compound mixed with the contact mass.
Union Carbide's European Patent No. 0028009 proposes to place the copper on the surface of silicon particles by mixing powders of two components and heating under a hydrogen atmosphere at about 1000.degree. C.
Elements other than copper have been recognized as having a favorable action on the rate or selectivity of the Rochow reaction. Thus, Wacker Chemie's U.S. Pat. No. 2,666,776 recommends the addition of a metal in the 5th or 8th group in the periodic table, particularly iron, nickel, cobalt or phosphorus to the silicon and copper in the contact mass.
Th. Goldschmidt's Patent DE 1165026 describes the use of silicon doped with phosphorus, arsenic, antimony, bismuth, indium, thallium and/or gallium. General Electric's U.S. Pat. No. 4,500,724 and Union Carbide's Patent EP 0191502 describe the use of a copper-zinc-tin mixture in defined proportions, in the contact mass.
Dow Corning's U.S. Pat. No. 4,602,101 proposes to add 25 to 2500 ppm of phosphorus to the contact mass, in the form of elementary phosphorus or compounds such as phosphides. Dow Corning's Patents EP 0272860 and 0273635 recommend additional phosphorus, either through raw materials used in the manufacture of silicon by carbothermy in an electric arc furnace, or by addition in the refining ladle.
The applicant's European Patent No. 0494837 describes a metallurgical silicon powder with a low surface oxidation, with a surface layer of silica less than 2 nm thick and particularly obtained by grinding under a very slightly reactive atmosphere in the presence of oil. This silicon can significantly improve the chloromethylation reaction rate.
On the subject of grinding techniques, Mitsubishi Metal's Japanese Patent application No. 055111 describes grinding of silicon in a ball grinder with the addition of 0.1 to 20% of a copper compound, for example a cupric or cuprous chloride or oxide, in order to prevent silicon from sticking on the grinder walls.